The present invention relates to non-aqueous electrolyte secondary batteries using non-aqueous electrolyte with lithium ion conductivity where material capable of occluding and emitting lithium is used as the negative active material and/or positive active material. In particular, it relates to novel negative active material and positive active material which provide novel secondary batteries of high voltage and high energy density having a long cycle service life with a graded charge/discharge characteristic.
A non-aqueous electrolyte battery, which uses lithium as a negative active material, has advantages such as lower self-discharge, high reliability for a long time, high voltage and high energy density, and so forth, and is widely utilized in power sources for a memory back-up, camera and the like as a primary battery. However in the recent years, with the remarkable development of portable type electronic devices and communication equipment and the like, there have been proposed various kinds and types of equipments which require large current output to the batteries as a power supply, thus secondary batteries of high energy densities capable of recharging and re-discharging are now in strong demand from the view point of economical efficiency, and compact size and light weight of the devices. For this reason, research and development for increasing the secondary batteries from among the non-aqueous electrolyte batteries having a high energy density are actively performed and a part thereof is now in practical use. However, the energy density, charge and discharge cycle life time, and reliability are still unsatisfactory.
Conventionally, the positive active material constituting the positive electrode of a secondary battery of such kind includes three kinds of types due to charge and discharge reaction profiles as undermentioned. The first type is one in which lithium ion (cation) only moves in or out of spaces between layer-to-layer, lattice positions, or gaps among lattices of the crystal depending on intercalation and deintercalation reactions and the like as seen in metal chalcogenides such as TiS.sub.2, MoS.sub.2, and NbSe.sub.3, and in metal oxides such as MnO.sub.2, MoO.sub.3, V.sub.2 O.sub.5, Li.sub.x CoO.sub.2, Li.sub.x NiO.sub.2, and Li.sub.x Mn.sub.2 O.sub.4, and in like cases. The second-type is one in which mainly anion only stably moves in and out by means of doping and undoping reactions as seen in conductive polymers such as polyaniline, polypyrrole, polyparaphenylene. The third type is one in which, lithium cation and anion can together move in or out as seen in graphite intercalation compounds and conductive polymers such as polyacene and the like (intercalation, deintercalation or doping, undoping and the like).
On the other hand, for the negative active material constituting the negative electrode of the battery of this kind, the basest electrode potential is provided in case of using metal lithium independently, and correspondingly the battery combined with the positive electrode using the positive active material as described above advantageously has the highest output voltage with a high energy density. However in this case, the problem arises because dendrite or passive compounds are generated on the negative electrode depending on charge or discharge to produce considerable deterioration due to charge and discharge and to shorten the cycle service life time. To solve this problem there are proposed various utilizations of materials as a negative active material; namely, (1) alloys obtained by combining lithium with other metals such as Al, Zn, Sn, Pb, Bi, and Cd; (2) intercalation compounds or insertion compounds where lithium ion is incorporated into the crystal structure of inorganic compounds such as WO.sub.2, MoO.sub.2, Fe.sub.2 O.sub.3, TiS.sub.2, and the like, graphite and carbonaceous materials obtained by baking organic material; and (3) conductive polymers such as polyacene, polyacetylene and the like in which lithium ion is doped.
However in general, in case where the negative electrode using material capable of occlusion and emission of lithium ion (other than metal lithium described above as the negative active material) is combined with the positive electrode using the positive active material described above to produce the battery, then the electrode potential of these negative active materials is nobler than the electrode potential of metal lithium, and a drawback therefore arises in considerably lowering an operating voltage of the battery compared to the case of independently using the metal lithium as a negative active material. For example, the operating voltage is lowered by 0.2 to 0.8 V in case of using an alloy of lithium and metals such as Al, Zn, Pb, Sn, Bi, and Cd, by 0 to 1 V for carbon--lithium intercalation compounds, and by 0.5 to 1.5 V for lithium ion insertion compounds such as MoO.sub.2, WO.sub.2 and the like.
Since elements other than lithium are involved as negative electrode constituent elements, the capacity and energy density per unit volume and unit weight are considerably lowered correspondingly.
In addition, when using an alloy of lithium and other metals in (1) described above, there are such problems that the utilization efficiency of lithium is low during charge and discharge, and the cycle life is short due to occurrence of cracks in the electrode to generate splits on account of repeated charge and discharge and the like. In the lithium intercalation compound or insertion compound in case (2), deteriorations such as decay of the crystal structure and generation of irreversible material are generated by excess charge and excess discharge, and the electrode potential is high (noble) in many cases, which results in a drawback of reducing an output voltage of the battery. In the conductive polymer in case (3), there is such a problem that the charge and discharge capacity, in particular, the charge and discharge capacity per unit volume, is small.
For these reasons, to obtain a long cycle life secondary battery having a graded charge and discharge characteristic with high voltage and high energy density, there is required a negative active material having a larger effective charge and discharge capacity, i.e., the amount capable of reversible occlusion and emission of lithium ion simultaneously with a lower (baser) electrode potential with respect to lithium without deterioration of crystal structure decay and irreversible substance generation due to the occlusion and emission of lithium ion at charge and discharge.
On the other hand, in the positive active material, the first type has a drawback in that a considerable deterioration arises due to crystal deintegration and irreversible substance generation on excess charging and excess discharging although its energy density is larger. To the contrary, in the second and third types, the charge and discharge capability, in particular, disadvantageously the charge and discharge capacity, and energy density per unit volume are significantly smaller.
Therefore, to obtain a high capacity and high energy density of a secondary battery having an upgraded excess charging characteristic and excess discharging characteristic, there is required a positive active material in which a larger amount of lithium ion can reversibly be occluded and emitted without crystal deintegration and irreversible substance generation due to the excess charge and excess discharge.